1. Field of the Invention
The present invention relates to an image forming apparatus and image forming process suitable for a copier, printer or facsimile machine.
2. Description of the Related Art
In the related art, in an image forming process comprising a step wherein a latent electrostatic image formed on a photoconductor was developed by a developer comprising a toner, a developed toner image was transferred to a transfer medium such as paper or the like, and the photoconductor was cleaned to prepare it for the next image forming operation, many proposals were made to improve image quality, make the image forming apparatus more compact, save energy, speed up the process and reduce costs.
For example, Japanese Patent Application Laid-Open (JP-A) No. 09-244292 proposes a method wherein, by coating a polyurethane wax surface adhering to toner particles with additive microparticles so that the surface is suitably exposed, the opportunities for direct contact between the wax and the photoconductor surface are reduced. According to this proposal, photoconductor filming due to the wax is suppressed, it is easy to make the apparatus more compact, reduce costs and simplify the process, and a toner is obtained without filming due to wax or additive separation and without loss of fluid properties due to crushing of the toner.
In Japanese Patent Application Publication (JP-B) No. 3000401, to improve cleaning properties of the photoconductor surface in the operations after developing, fatty acid removal type carnauba wax and/or dimethyl silicone oil having a dynamic viscosity factor of 30 centipoises to 60,000 centipoises and a molecular weight of 2,000 or less is supplied as a material to reduce the frictional coefficient of the photoconductor surface.
In JP-B No. 2675974, to make the unevenness of the toner particle surface and hardness of the toner particles suitable, magnetic toner particles containing 1% to 10% of a polyalkylene resin having a weight average molecular weight of 3,000 to 80,000 relative to toner resin, and a silica microparticle additive, these magnetic particles having a dynamic frictional coefficient in a range between 0.20 which is sufficient to polish the photoconductor surface and 0.50 which does not scratch the photoconductor surface, are described.
In JP-B No. 08-3656, a toner comprising a magnetic toner A and a colorless toner B, wherein the toner B has a dynamic frictional coefficient in a range between 0.20 which is sufficient to polish the photoconductor surface and 0.60 which does not scratch the photoconductor surface, the dynamic frictional coefficient of toner A being less than that of toner B, is described.
In JP-B No. 06-82226, a latent electrostatic image formed on a OPC photoconductor having a surface hardness of 10 g to 100 g is developed to form a toner image using a toner containing polyalkylene microparticles and hydrophobically-treated silica having an average particle diameter of 3.0 μm or less, the toner having a dynamic frictional coefficient of 0.15 to 0.65, and cleaning is performed.
In JP-A No. 11-95477, a toner comprising toner particles containing a binder resin, colorant, vegetable wax of melting point 66° C. to 86° C. and/or polyethylene wax of melting point 80° C. to 140° C., and an additive, the dynamic frictional coefficient of the toner being 0.12 to 0.30, is described.
In JP-A No. 2000-105484, a toner containing a three-dimensional cross-linked polyester resin binder whereof the static frictional coefficient is reduced to 0.4 or less by a wax, and is suitable for flash fusing fixing, is described.
In JP-A No. 2001-5220, a color toner wherein the difference between the maximum dynamic frictional coefficient and minimum dynamic frictional coefficient of 4 colored toners is adjusted to 0.2 or less, is described.
In recent years, due to limited office space, there has been a demand to make copiers and composite devices more compact, and also to make the units comprising these devices more compact.
Small photoconductor drums installed in actual machines have an outer diameter of 20 mm to 30 mm, but in general, as it is required to perform charging, exposure, developing, transferred, cleaning and discharging steps, it is necessary to install units to provide these functions around the photoconductor of an image forming apparatus using the electrophotographic method. However, in the present technology, there is a limit to the compactness of the units surrounding the photoconductor. In addition to the various units mentioned above, it is also required to install separating claw and reflecting photosensors (hereafter, “P sensors”) to control image density.
The P sensor detects the toner amount adhering to the photoconductor surface, and feeds this back to toner supplementation amount control. When controlling image density on a transfer paper, as the toner amount can be detected closer to the final step of the electrophotographic process, it is therefore an effective means of achieving stable image density.
When this P sensor detects the toner image developed on the photoconductor surface, the detection must be performed between the developing step and cleaning step which are performed around the photoconductor. In other words, the aforesaid P sensor must be installed in the vicinity of the transfer step, and it must be installed without obstructing the transfer paper transport path.
One means of installing the P sensor within these limitations is for example a remote P sensor. This remote P sensor is separated from the photoconductor surface, the detection target, by approximately 20 mm, and is therefore installed further away from the photoconductor than the 3 mm to 5 mm of the prior art near type. This has a major advantage in that, provided the P sensor has a sufficient light path for emitting and receiving light, as it is not necessary to install the P sensor in the vicinity of the photoconductor, the limited space of the image forming apparatus in which the small diameter photoconducting drum is installed can be effectively utilized.
The control of the P sensor will now be described.
First, the light emission amount is varied so that the sensor output (Vsg) is 4.0V compared to the background part of the photoconductor to which toner is not adhering. In practice, to adjust the light emission amount of the P sensor installed in the image forming apparatus, the current flowing through the P sensor light-emitting element is controlled by PWM, and this PWM value is automatically varied so that Vsg adjustment is terminated when Vsg=4.0V. Subsequently, it is fixed at the adjusted PWM value on the next occasion Vsg adjustment is performed.
Vsg adjustment is performed for example when the main switch on the image forming apparatus is switched ON, when the apparatus is recovering from the pre-heating mode, and when a copy operation is terminated after a preset number of copies have passed through the machine.
In general, the toner adhesion amount on the photoconductor is detected by the P sensor after every 100 copies have been made, and the toner supplementation amount is determined by an output ratio (Vsp/Vsg) of the output (Vsp) when a toner adhesion pattern for the P sensor is detected, and the background part detection output (Vsg).
As the light source of an economical P sensor, a phototransistor or photodiode may be used, but unlike laser light, the light from this diverges to some extent. Therefore, although normally-reflected light is mainly received, some diffuse reflected light is also received.
However, in this image forming apparatus using an electrophotographic method, the photoconductor is in contact with a large number of components such as a developer, developer inlet seal to prevent toner scatter, cleaning blade, cleaning fur brush, cleaning inlet seal, separating claw, discharge roller and transfer roller, and there is constant friction with these components during the copying process. As a result, when the copy operation is performed repeatedly, the photoconductor surface gradually wears down.
Of those members in contact with the photoconductor, the cleaning blade is in contact with the photoconductor under a constant pressure due to its function of eliminating toner adhering to the photoconductor surface, so it makes a large contribution to the wear of the photoconductor surface. In this regard, it has been proposed to use a toner containing a wax which reduces the frictional coefficient of the photoconductor (JP-A No. 09-244292), or to supply a wax or silicone oil to the photoconductor surface (JP-B No. 3000401).
If the wax amount which leaks from the toner on the cleaning blade is uneven, some parts of the photoconductor will have high wear and others will have low wear, and the photoconductor will wear unevenly as described above.
Also, to prevent streaks due to interposition of foreign material, an image forming apparatus has been proposed wherein a cleaning blade is vibrated in the photoconductor axis direction. In this image forming apparatus, local wear is prevented due to the vibrating mechanism and the photoconductor wears more evenly, but it is difficult to install in an image forming apparatus where low-cost and small space are desired.
If such a photoconductor with uneven wear is used, an uneven density image with vertical striations may be obtained in the case of halftone images, etc.
This is because the distant P sensor receives a greater proportion of normally-reflected light than a near sensor, and the attenuation factor of the received light relative to the emitted light is also high. Therefore, if a Vsg adjustment of the P sensor is made using a photoconductor of uneven wear, the normally-reflected light remarkably decreases compared to a photoconductor which has no wear or a photoconductor which has even wear, so the emitted light amount from the P sensor, i.e., the PWM value, must be largely increased. If the large increment of this PWM value exceeds the limit, the Vsg adjustment of the P sensor will be unsatisfactory.
The aforesaid defect is due to uneven wear of the photoconductor surface over time, and is affected by the wax which has leaked from the toner.
The wax added to the toner melts in the step which fixes the toner image on the transfer paper transferred in the transfer step, and has the effect of helping it to separate from the fixing roller so that an offset image is not produced. Hence, the offset tolerance during fixing increases the larger is the wax amount in the toner, but on the other hand, as the wax amount which gradually flows out onto the toner surface due to hazards such as heat or pressure imparted to the toner in the developer or cleaning part increases, the more is the addition amount in the toner, the more is the uneven wear of the photoconductor surface.
The wax which leaked onto the toner surface is spent on the carrier surface, and due to the deterioration of charge, toner adheres to the non-image part of the photoconductor causing background soiling.
Further, toner microparticles where wax has leaked onto the surface are not easily transferred, are scraped by the cleaning blade, and return again to the developer part via a recycle path which makes image deterioration even worse.
The wax smears on the toner surface depend on the wax amount, and are also largely affected by the wax dispersion diameter in the toner.
For example, if toner is subjected to pressure due to a physical force such as by stirring the developing part, due to the hazard resulting from the heat or pressure produced at that time, wax leaks onto the toner surface so as to cause “bleed out”. This phenomenon tends to occur more easily, the larger is the wax dispersion diameter.
Further, the wax smears on the toner surface are affected by the wax addition amount to the toner, and are also largely affected over time by the process conditions of the copy machine. For example, when the developer part is stirred, the proportion of wax leaking onto the toner surface due to the physical stress of pressure and heat increases. If this wax smear amount is unevenly distributed, some parts of the photoconductor will have a high frictional coefficient and some parts will have a low frictional coefficient, so the photoconductor will have uneven wear, and an image of uneven density with vertical striations may appear in the case of halftone images, etc.
Users of low speed machines having a system line speed of 100 mm/sec to 200 mm/sec frequently output only one copy or print, and the developer stirring time during developing becomes larger compared to the number of copies and prints.
It is known that in general, when the rotation (stirring) time of the developer is longer in terms of unit copies, stress such as heat or pressure acting on the developer increases. In other words, in the case of a mode where one copy is made from one original, 2 to 6 times the rotation time is required compared to the developer time per sheet when continuous copies are made, and therefore a very large heat stress acts on the developer. This phenomenon is particularly frequent in the case of users of low speed machines having a system speed of 100 mm/sec to 200 mm/sec.
For example, in the case of a machine which can produce 27 copies per minute, for continuous copies, approximately 3 seconds is required per sheet. However, for making one copy, the developer unit rotates for approximately 7.5 seconds. The reason for this is that even if the motor is running, transfer paper is prepared and developing is completed after the switch is switched ON, due to the transfer, fixing and paper ejection steps, a long time is required. This could be dealt with by stopping the operation of the developing part immediately after developing is finished, but if the photoconductor is rotating, carrier adhesion takes place which is a problem. In view of the time for which the P sensor functions in terms of unit copies, 2 to 6 times the developing rotation time is presently required. As a result, the heat and pressure stress acting on the developer increases as described above, and this shortens the life of the developer.
Object and Advantages
It is a first object of the present invention to provide an image forming process and image forming apparatus wherein, in an image forming apparatus designed for low-cost and small space, to suppress uneven wear of a photoconductor over time, a developer having an appropriate toner dynamic frictional coefficient which is affected by surface wax is combined with the most suitable processes to eliminate uneven wear due to abrasion of the photoconductor, background soiling and unevenly dense images with vertical striations, such as in the case of halftone images or the like.
It is a second object of the present invention to provide an image forming process and image forming apparatus wherein, in an image forming apparatus designed for low-cost and small space, to suppress uneven wear of a photoconductor over time, a developer having an appropriate wax dispersion diameter in a toner which is affected by surface wax is combined to eliminate uneven wear due to abrasion of the photoconductor, background soiling and unevenly dense images with vertical striations, such as in the case of halftone images or the like.